C O ] 1 0 A pr 2 01 3 Gradients of Stellar Population Properties and Evolution Clues in a Nearby Galaxy M 101

Multi-band photometric images from ultraviolet and optical to infrared are collected to derive spatially resolved properties of a nearby Scd type galaxy M 101. With evolutionary stellar population synthesis models, two-dimensional distributions and radial profiles of age, metallicity, dust attenuation, and star formation timescale in the form of the Sandage star formation history are obtained. When fitting with the models, we use the IRX-AFUV relation, found to depend on a second parameter of birth rate b (ratio of present and past-averaged star formation rate), to constrain the dust attenuation. There are obvious parameter gradients in the disk of M101, which supports the theory of an “inside-out” disk growth scenario. Two distinct disc regions with different gradients of age and color are discovered, similar to another late-type galaxy NGC 628. The metallicity gradient of the stellar content is flatter than that of H ii regions. The stellar disk is optically thicker inside than outside and the global dust attenuation of this galaxy is lower, compared with galaxies of similar and earlier morphological type. We highlight that a variational star formation timescale describes the real star formation history of a galaxy. The timescale increases steadily from the center to the outskirt. We also confirm that the bulge in this galaxy is a disk-like pseudobulge, whose evolution is likely to be induced by some secular processes of the small bar with relatively young age, rich metal, and much dust. Subject headings: Galaxies: individual(M 101) — Galaxies: evolution — Galaxies: photometry — Galaxies: bulge Center for Astrophysics, University of Science and Technology of China, Hefei 230026, China; linlin@mail.ustc.edu.cn, zouhu@nao.cas.cn, xkong@ustc.edu.cn Key Laboratory for Research in Galaxies and Cosmology, Chinese Academy of Sciences, Hefei 230026, China National Astronomical Observatories, Chinese Academy of Sciences, Beijing 100012, China